1. Field of the Invention
This invention relates to a method and apparatus for converting a standard fixed bed pickup truck to a tilting bed truck, and more particularly, to a geometrically improved lifting hoist mechanism for converting a fixed bed pickup truck to a tilting bed truck such that the relationship of the pickup bed to the passenger cab remains unchanged after the conversion.
2. Prior Art for
The advantages in having a truck with the capability of selectively tilting the truck bed or hauling box to facilitate removal of materials transported thereon has resulted in a number of systems for providing such a feature. The systems heretofore used have been primarily of two types. The first is characterized by a truck bed hinged at the rear of the truck frame and hoisted or tilted by the actuation of a hydraulic ram or other variable length arm connected directly between the forward end of the truck bed and truck frame. In order to accommodate the lifting ram and permit the truck bed to be positioned adjacent the truck frame when in the horizontal position, the lifting ram in these systems is normally exposed either within or without of the hauling bed, thus introducing the substantial disadvantage of having the lifting ram exposed when not in use. Alternatively, the lifting ram is positioned below the truck bed with the bed substantially raised above its normal design position or with the ram so positioned as to introduce very inefficient leverage as it functions to tilt the truck bed.
A second type of lifting mechanism has been employed wherein a hydraulic ram is used to extend a folded linkage consisting of two or three interconnected members. By extending the uppermost member, the truck bed is raised to the tilted position. Examples of this type of system are found in U.S. Pat. No. 3,617,090, issued to Samuel E. Huffman and U.S. Pat. No. 3,043,629, issued to D. H. Schlueter, et al. In each of these configurations, the truck bed is tilted by extending the upper arm by use of a hydraulic ram acting between the lower arm and the upper arm. While this configuration introduces the advantage of a folding scissor-type lift mechanism which collapses below the truck bed, lifting of the bed by extending the upper arm against the underside of the truck bed introduces substantial loading including stresses due to a considerable bending moment applied to the upper arm. Further, the prior art units fail to disclose a method for simply and quickly altering a fixed bed truck for the installation of tilting mechanisms without altering the relationship between the truck bed and the frame and passenger cab of the truck. Moreover, the prior art systems have failed to disclose a simple conversion system which does not require extensive alteration to the truck frame or bed and which does not effect either the structural or operating vehicle components such as the drive shaft, muffler and other original parts in converting the truck from a fixed bed to a tilting bed vehicle.